Method for horizontal continuous casting of magnesium slab or magnesium alloy slab and apparatus therefor

ABSTRACT

A molten Mg or Mg alloy is supplied into a mold, which is sheltered from the surroundings, from a pool in a tundish, which is shielded from the surroundings. An Mg slab or Mg alloy slab in the mold is drawn out to the outside from an outlet of the mold. A cooling medium is jetted onto the slab on an outer side of and in a vicinity of the outlet to cool the slab. A sealing gas is sprayed over an entire width of a free surface of the molten Mg or Mg alloy on an inner side of and in the vicinity of the outlet, to seal a portion above the entire width of the free surface of the molten Mg or Mg alloy, whereby preventing oxygen from flowing into the mold from the portion and preventing the cooling medium from running back to the mold side.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for horizontally drawing-outtype (horizontal) continuous casting for horizontally drawing out andcontinuously casting a magnesium slab (hereinafter, referred to as Mgslab) or a magnesium alloy slab (hereinafter, referred to as Mg alloyslab), and relates to an apparatus for horizontally drawing-out type(horizontal) continuous casting which is used for the method.

2. Description of the Related Art

Recently, recyclable magnesium alloys (Mg alloys), which are light inweight and have high strength, have been used for cases and frames ofhousehold appliances such as personal computers and portable telephonesin many cases. Further, in recent years, the alloys have been applied toautomobile parts in many cases for the reduction in weight of anautomobile in the West.

A conventional method for producing a laminate plate is generally amethod in which a slab manufactured through casting (slab manufacturedby a continuous casting method) is subjected to rolling. In order toproduce a laminate plate D from a slab B having crystal grain boundariesA without the occurrence of coming-off and edge cracks C of a surface asshown in FIG. 14, a large number of (in general, over 25) processingsteps are required, including grinding of a surface layer of a slab andcutting of side edges thereof, and further, hot rolling, cold rolling,and heat treatment. Accordingly, this conventional method has a problemin that the cost of a rolling plate member is high. In addition, in thecase of a slab with poor workability which is made of magnesium (Mg) orMg alloy, it is extremely difficult with the above method to plasticallywork the slab into a laminate plate without the occurrence of coming-offor edge cracks of a surface thereof.

Examples of existing Mg alloys include cast products made by die castingand Thixomolding and products made by extrusion. On one hand, the caseof the cast products involves low productivity because of a batch typeprocess; on the other hand, the case of extrusion also involves lowproductivity despite a semi-batch type process. This is one of thecauses of high cost. In any manufacturing method, the following problemsexist. That is, in the case of a slab or billet produced by conventionalcontinuous casting, the crystal grain boundaries A are generated whichextend from a surfaces of a cast product or an extrusion billet to aninterior portion thereof as shown in FIG. 13, and the boundaries remainas traces in the product. Further, a hydrogen gas and impurities J in amolten metal gather in the product, and shrinkage holes I and gasporosities are generated due to solidification shrinkage. This leads tomany inferior products, a low yield, and low reliability of the productquality. Moreover, there is a limitation to the size of a die for Diecasting and Thixomolding even if the size is to be increased. That is,there is a limitation to the maximum dimension of the product that canbe produced. Thus, a product with a large dimension is hard to beproduced.

As a method of manufacturing a strip of a nonferrous metal such as an Al(aluminum) alloy or Sn (tin) alloy, a horizontal continuous castingmethod has been developed in the prior art. In the method, as shown inFIG. 15, a casting molten metal E is supplied into a mold L having agutter shape, which has been heated to a temperature not lower than asolidification temperature of the casting metal; a metallic molding(metallic strip) G, which is formed in the mold L, is drawn out by adummy member; and at this point, the drawn-out metallic molding G issprayed with a cooling water H from an upper portion of the mold L,thereby cooling the metallic molding G. In this case, an upper openingportion of the mold L is provided with an air curtain member K, and anair curtain is made from a gas jetted from the air curtain member,whereby the cooling water H, droplets thereof, and the like areprevented from entering the molten metal E in the mold L. This method iscommonly called an OSC method. Symbol 1 in FIG. 15 denotes a gap betweena bottom surface of the mold L and the metallic molding G which isgenerated by slanting the bottom surface of the mold L downward towardthe outlet side (refer to JP 6-88106 B).

In some conventional strip manufacturing apparatuses, as shown in FIG.16, a sheltered member M shields a part of an upper surface which iscloser to an inlet of the heating mold L; a gas supply pipe N isperpendicularly inserted into the sheltered member M; an inert gas isjetted from the pipe to form an air curtain; the air curtain shields agap between an upper surface of a molten metal O in the heating mold Land a back surface of the sheltered member M, thus preventing watervapor, which is generated by spraying the cooling water H onto thedrawn-out metallic strip G, from entering the molten metal side tothereby avoid oxidation of cupper, silicon, stainless steel, and thelike (refer to JP 4-125046 ).

The method of manufacturing a strip of a nonferrous metal as disclosedin JP 6-88106 B (the OSC method) is superior to conventionalmanufacturing methods in that formation of a solidified shell on aninner wall surface of the mold is prevented because the inside of themold is heated to a temperature not lower than a solidificationtemperature of a casting metal, and thus, a strip can be obtained whichconsists of a complete unidirectional solidification structure in whichcrystals are grown only in a casting direction. Further, the air curtainis formed to prevent the cooling water H, droplets thereof, and the likefrom entering the molten metal E side in the mold L, and this providesan oxidation prevention effect. However, the air curtain member K islocated apart from an outlet f of a tundish F. Thus, air enters the moldL from an upper surface opening portion P of the mold L between the aircurtain member K and the outlet f, which inevitably results in oxidationof the molten metal in the mold L. Therefore, in the OSC method, in thecase of Mg or Mg alloy which is easy to be oxidized, a surface of an Mgslab or Mg alloy slab burns at the same time the slab is drawn out fromthe mold L, and the surface becomes black due to the generation of alarge gray and black oxide. Accordingly, it has been difficult toproduce a slab of which surface does not become black. Further, whenoxygen flows into the mold L, a molten Mg burns through reaction withoxygen in the air; when the combustion contacts with the cooling water,the water is decomposed to generate hydrogen and oxygen, possiblyleading to an explosion. Furthermore, in manufacturing of an AZ31B slabwith the OSC method, a surface thereof is uneven as shown in FIG. 12.Thus, it has been impossible to produce a smooth plate-like slab.

In the strip manufacturing apparatus of Japanese Utility ModelApplication Laid-open No. Hei 4-125046, the inert gas is jetted from thegas supply pipe N to form the air curtain; the air curtain shields thegap between the upper surface of the molten metal O in the heating moldL and the back surface of the sheltered member M, thus preventing watervapor, which is generated by spraying the cooling water, from enteringthe molten metal side to thereby avoid oxidation of cupper, silicon,stainless steel, and the like. However, the gas supply pipe N is locatedperpendicular to the molten metal O in the heating mold L, and thus, thegas jetted from the gas supply pipe N is not easily dispersed to theoutlet side of the heating mold L. Therefore, it has been difficult tosufficiently prevent oxygen from entering the mold L from the outletside of the heating mold L even if a large quantity of expensive inertgas is flown into the mold L. According to the experiment conducted bythe inventors of the present invention, it has been found that themanufacturing apparatus cannot be used for casting of Mg or Mg alloywhich is easy to be oxidized and which is attended by the danger ofexplosions through contact with water. Further, the gas supply pipe N isa mere pipe. Thus, a jetting area of the gas jetted from the outlet isnarrow, and therefore, the pipe cannot be used for manufacturing of anMg slab or Mg alloy slab which has a wide width (for example, a width of100 mm or more).

In either of the cases of JP 6-88106 B and Japanese Utility ModelApplication Laid-open No. Hei 4-125046, since the cooling water issprayed onto the metallic strip G on the mold L to cool the metallicstrip G, not only the metallic strip G but also the mold L is cooled.Thus, it is necessary to heat the mold in advance to a highertemperature than needed while taking into account this cooling amount.The mold needs to be heated to a temperature higher than a melting pointof a pure metal or a solidification temperature of a metal alloy byapproximately 300° C. to 350° C. Such a high temperature leads to thelarge transformation of the mold due to thermal expansion and theshorter life of the mold or mold heater R. Thus, these components needto be exchanged frequently (for example, 1 batch: every 29 kg) . As aresult, mass production has been difficult to achieve in actuality.Further, in the case where the mold L is formed of a metal, when the Mgslab or Mg alloy slab is cast, a center portion in a width direction ofa slab S is curved upward as shown in FIG. 17A. As a result, it isimpossible to produce the Mg slab or Mg alloy slab S with a uniformthickness in the width direction. In the case of a markedtransformation, as shown in FIG. 17B, the molten Mg or Mg alloy, whichis to be supplied to the mold L, is separated to both the side surfacesides of the mold L from the upper curved portion of the Mg slab or Mgalloy slab S, as a result of which it is impossible to produce the Mgslab or Mg alloy slab with a wide width. Besides, the Mg slab or Mgalloy slab S that has been separated on both the sides of the mold L hasan uneven thickness, which makes rolling in subsequent workingdifficult. In either of the cases of the OSC method (JP 6-88106 B) andthe OSC method with a sheltered board (Japanese Utility ModelApplication Laid-open No. Hei 4-125046), when casting of the metal suchas Mg is performed, the temperature of the mold needs to be set prettyhigher than the melting point of the casting metal (Mg) or thesolidification temperature of the casting alloy (Mg alloy)(approximately m.p.+300 to 350 K) in consideration of a fact that thelatter half of the heat molding process involves cooling with thecooling water. Thus, under the condition of such a high temperature, theinsulation resistance values (MΩ) of a heater panel, which supports andholds an electric heater of a heating element for heating a mold, and arefractory and a heat insulating material, which surround the peripheryof the heater to avoid heat radiation of the heater to the outside ofthe mold, are vastly reduced (normally, reduced from a value not lessthan about 2 MΩ to a value not more than about 0.1 MΩ). Therefore, theleak of a heater temperature control current occurs. This results in aproblem in that continuous casting is impossible to achieve.

Against the background described above, there has been an increasedmarket demand for expanded products formed through press working moldingof Mg laminate plates or Mg alloy laminate plates with high productivityand highly reliable product quality. However, a safe and stable castingmethod of an Mg slab or Mg alloy slab, which is a raw material for alaminate plate, has not been sufficiently established. Thus, at thepresent, domestic mass production of slabs has not been realized. Thereis only one commercial supply maker of slabs in the U.S. Accordingly, ithas been strongly desired that the domestic supply of slabs be realizedas soon as possible also in view of the necessity of lowering highcosts.

SUMMARY OF THE INVENTION

The inventors of the present invention have made effortful studies of amethod of easily manufacturing an Mg slab or Mg alloy slab and amanufacturing apparatus used for the method in a horizontal continuouscasting method including an OSC method. From the studies, they succeededin stable manufacturing of an Mg slab or Mg alloy slab which has nocrystal grain boundary (A in FIG. 14), the crystal grain boundary beingformed on a surface or an end portion toward an interior portion of theslab and being a main cause of occurrence of coming-off and edge cracksof the surface in hot or cold rolling working, which does not have innerholes, gas porosities, inclusions, and the like, and which is notoxidized to become black, without the danger of explosions.

According to a method for horizontal continuous casting of an Mg slab orMg alloy slab of this patent application, a molten Mg or Mg alloy issupplied from a pool in a tundish, which is shielded from surroundings,into a mold, which communicates with the pool and is shielded from thesurroundings. Then, the Mg slab or Mg alloy slab formed in the mold isdrawn out to the outside from an outlet of the mold. After the mold isdrawn out, a cooling medium is jetted onto the Mg slab or Mg alloy slabon the outer side of the mold outlet and in the vicinity of the outletto cool the Mg slab or Mg alloy slab. A sealing gas such as an inertgas, nitrogen gas, or incombustible gas is jetted toward the outlet sideof the mold over the entire width of a free surface of the molten Mg orMg alloy on the inner side of the mold outlet and in the vicinity of theoutlet. The gas seals a portion above the entire width of the freesurface of the molten Mg or Mg alloy in the mold to prevent oxygen fromflowing into the mold. As a result, in the case where the cooling mediumis liquid, the liquid, droplets, spray, and the like are prevented fromrunning back to the mold side. A surface temperature of the mold is setat a temperature exceeding or not exceeding a melting point of Mg or asolidification temperature of an Mg alloy. In the case of thetemperature not exceeding the melting point or solidificationtemperature, a bottom surface and an inner wall surface of the mold arecovered with a thin solidified shell formed of the molten Mg or Mg alloysupplied from the pool in the tundish. The unsolidified molten residueis solidified on the solidified shell to form the Mg slab or Mg alloyslab. Further, between the mold outlet and the cooling medium jettingpart, a second sealing gas such as air, an inert gas, nitrogen gas, orincombustible gas is jetted onto the cooling medium jetting side overthe entire width of the Mg slab or Mg alloy slab, which has been drawnout from the outlet of the mold. The second sealing gas seals theportion above the entire width of the Mg slab or Mg alloy slab. In thecase where the cooling medium is liquid, the seal also prevents theliquid, droplets, spray, and the like from running back to the mold side(in a two-staged manner). Further, when the Mg slab or Mg alloy slabformed in the mold is to be drawn out from the outlet of the mold, a tipend of the molten Mg or molten Mg alloy is made to contact with a dummymember at the time of the start of drawing-out. The Mg slab or Mg alloyslab is drawn out by drawing out the dummy member. After being drawnout, the Mg slab or Mg alloy slab is drawn out by a drawing devicewithout the use of the dummy member. Moreover, the sealing gas is jettedalong the free surface of the molten Mg or Mg alloy; and the secondsealing gas is jetted to the cooling medium jetting side along an uppersurface of the Mg slab or Mg alloy slab.

An apparatus for horizontal continuous casting of an Mg slab or Mg alloyslab according to this patent application is provided with: a shelteredboard which covers an area extending from a portion above a pool in atundish to a portion above a mold outlet to shield the area from theoutside; a cooling medium jetting tool which is provided on the outerside of the mold outlet or at an outlet end of the mold and which jets acooling medium onto the Mg slab or Mg alloy slab, which has been drawnout from the outlet, on the outer side of the mold outlet and in thevicinity of the outlet to cool the slab; and a gas jetting tool which isprovided on the inner side of the mold outlet and which jets a sealinggas such as an inert gas, nitrogen gas, or incombustible gas on the sideof a free surface of a molten Mg or Mg alloy to seal a portion above theentire width of the free surface of the molten Mg or molten Mg alloy inthe mold. The sealing gas jetted from the gas jetting tool seals theabove-mentioned portion to prevent oxygen from flowing into the moldfrom the portion. In the case where the cooling medium is liquid, thesealing gas prevents the liquid, droplets, spray, and the like fromrunning back to the mold side. Between the outlet of the mold and thecooling medium jetting tool, a second gas jetting tool is provided whichjets a second sealing gas such as air, inert gas, nitrogen gas, orincombustible gas on the Mg slab or Mg alloy slab drawn out from theoutlet mold to thereby seal a portion above the entire width of the Mgslab or Mg alloy slab. In the case where the cooling medium is liquid,the seal also prevents the liquid, droplets, spray, and the like fromrunning back to the mold side. The second gas jetting tool is providedat an angle at which the second sealing gas is jetted along the Mg slabor Mg alloy slab. A drawing device, which draws out the Mg slab or Mgalloy slab from the outlet, is provided on the outer side of the moldoutlet. The mold is provided with a heating element so as to be heated,or is not provided with the heating element so as not to be heated. Inthe case where the heating element is provided, the operation of theheating element is made switchable between on and off.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are a schematic view showing a cross section of anapparatus for horizontal continuous casting of an Mg slab or Mg alloyslab according to the present invention and a sectional view of a dummyportion, respectively;

FIG. 2 is a schematic view showing a cross section of an outlet portionof a mold having a built-in heater in the apparatus for horizontalcontinuous casting of an Mg slab or Mg alloy slab;

FIGS. 3A and 3B are a schematic view, as seen from above, of theapparatus for horizontal continuous casting of an Mg slab or Mg alloyslab, and a sectional view of a sight window and a hinge portionattached thereto, respectively;

FIG. 4 is a schematic view showing a cross section of an outlet portionof a mold in the case where the apparatus for horizontal continuouscasting of an Mg slab or Mg alloy slab is additionally provided with ajetting tool for air or gas;

FIG. 5 is a schematic view of an outlet end of the mold of the apparatusfor horizontal continuous casting of an Mg slab or Mg alloy slab;

FIG. 6 shows an appearance of an AZ31B magnesium slab manufacturedthrough horizontal continuous casting of an Mg slab or Mg alloy slab;

FIG. 7 shows a macrostructure of a section taken in a longitudinaldirection of the AZ31B magnesium slab;

FIG. 8 shows a macrostructure of a section taken in a width direction ofthe AZ31B magnesium slab;

FIG. 9 shows a microstructure of the section taken in the longitudinaldirection of the AZ31B magnesium slab;

FIG. 10 shows a microstructure of the section taken in the widthdirection of the AZ31B magnesium slab;

FIG. 11 shows a microstructure of a raw material of the AZ31B magnesiumingot (ingot);

FIG. 12 shows an appearance of the AZ31B magnesium slab produced by aprior art (OSC method);

FIG. 13 is an explanatory plan view for the formation of crystals (grainboundaries) in general continuous casting of a slab (slab producedthrough continuous casting) and a billet (billet produced throughcontinuous casting);

FIG. 14 is a schematic view showing crystal grain boundaries andinfluence at the time of rolling;

FIG. 15 is a schematic view showing a cross section of a castingapparatus of the prior art (OSC method: JP 6-88106 B);

FIG. 16 is a schematic view showing a cross section of another castingapparatus of the prior art (Japanese Utility Model Application Laid-openNo. Hei 4-125046); and

FIGS. 17A and 17B are a longitudinal sectional view in the case where amold and an Mg slab or Mg alloy slab are curved upward and alongitudinal sectional view in the case where the mold and the Mg slabor Mg alloy slab are extremely curved upward, respectively, in the caseof casting the Mg slab or Mg alloy slab with the casting apparatus ofFIG. 16.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(Embodiment 1 of an Apparatus for Horizontal Continuous Casting of an MgSlab or Mg Alloy Slab according to the Present Invention)

Prior to explanation of a method for horizontal continuous casting of anMg slab or Mg alloy slab according to the present invention, detaileddescription will be made of Embodiment 1 of an apparatus for horizontalcontinuous casting of an Mg slab or Mg alloy slab which is used for themethod with reference to the accompanying drawings. In the apparatus forhorizontal continuous casting of an Mg slab or Mg alloy slab which isshown in FIGS. 1A to 3B, a sheltered board 5 is provided to cover anarea ranging from a portion above a pool 2 in a tundish 1 to a portionabove an outlet 4 of a mold 3, thereby shielding the area from theoutside; a cooling medium jetting tool 7, which jets a cooling medium 18onto an Mg slab or Mg alloy slab 6 drawn out from the outlet 4 tothereby cool the slab 6, is provided on the outer side with respect tothe outlet 4 of the mold 3; and a gas jetting tool 8 is arranged on theinner side with respect to the outlet 4 of the mold 3, as shown in FIG.3A and FIG. 5.

Heaters 12 are built in a bottom wall 10 and a peripheral wall 11 of thetundish 1, but no heater 12 is built in the mold 3. The outlet side ofthe tundish 1 and the mold 3 each have a gutter shape (upwardly facingconcave shape) having an upper opening. The sheltered board 5 covers theupper opening portions, thereby covering the upper surfaces from theoutside. Thus, a molten Mg or Mg alloy 13 in the tundish 1 and the Mgslab or Mg alloy slab 6 in the mold 3 do not contact with the air. Acalcium silicate board, kaowool board, or the like can be used for thesheltered board 5. The mold 3 is provided horizontal to the bottom wall10 of the tundish 1, or provided such that the outlet side is inclineddownward by 1 to 3 degrees with respect to the inlet side, whereby themolten Mg or Mg alloy 13 in the tundish 1 flows to the outlet side ofthe mold 3 more smoothly.

Stainless steel (SUS 304 or 430 etc.) is suitably used for the mold 3,and it is preferable that a mold be used in which an inner surfacethereof has been subjected to molten aluminum plating. The mold 3 mayhave a built-in heating device such as the heater as in FIG. 2 or otherheating elements. When the mold is heated by the heating device,solidification of the molten Mg alloy, which is supplied from the poolin the tundish into the mold, progresses more slowly. Thus, at the timeof the start of drawing-out, a tip end of the molten Mg or Mg alloy canbe coupled with a dummy member relatively slowly with a margin of time.Also, the thickness of a solidified shell, which is generated along aninner wall surface of the mold, can be reduced. Further, when a surfacetemperature of the mold is raised to a temperature exceeding a meltingpoint of casting Mg or a solidification temperature of an Mg alloy,crystal grain boundaries, which are formed on a surface or an endportion side toward an inner portion, are not generated. Thus, the Mgslab or Mg alloy slab can be produced, of which lower surface and sidewall are smooth and which has no casting defect, without the generationof the solidified shell. Therefore, heating is desirably performed witha condition that the mold temperature within a temperature range closeto or exceeding the melting point of casting Mg or the solidificationtemperature of an Mg alloy (a range approximately from m.p.+70 K tom.p.+170 K). In casting of a metal such as Mg, for example, inductionheating or heating with a gas burner may be adopted as a casting heatingmethod instead of heating with an electric heater. However, the electricheater, which is more compact and has high stability in temperature, isdesirably used.

As shown in FIGS. 3A and 3B, a sight window 14 is formed at a tip end ofthe sheltered board 5 covered on the mold 3. The sight window 14 isformed by cutting out the tip end from the sheltered board 5. The sightwindow 14 is covered by a movable lid 15, and the movable lid 15 isopenably and closably attached to the sheltered board 5 by means of ahinge 16. When the movable lid 15 is opened, the Mg slab or Mg alloyslab 6 in the outlet 4 of the mold 3 can be seen. When the movable lid15 is closed, the sight window 14 is shut.

A slit-shaped nozzle, round pipe nozzle, or the like can be used for ajetting outlet 7 a (FIGS. 1A to 4) of the cooling medium jetting tool 7.The jetting outlet 7 a is arranged downward such that the cooling medium18 is jetted onto the Mg slab or Mg alloy slab 6 drawn out from theoutlet 4 of the mold 3. The width of the jetting outlet 7 a issubstantially the same as or slightly narrower than the width of the Mgslab or Mg alloy slab 6 as shown in FIG. 3A such that the medium isjetted all over the Mg slab or Mg alloy slab 6. The cooling mediumjetting tool 7 is supplied with the cooling medium 18 such as water orcooling gas from a cooling medium supply device (not shown).

In the gas jetting tool 8, a supply opening 8 a is located above a freesurface 13 a of the molten metal 13 in the mold 3; a sealing gas 19 suchas an inert gas, nitrogen gas, or incombustible gas is supplied from thesupply opening 8 a toward the free surface 13 a side to form a gascurtain 21 for sealing a gap 20 between the free surface 13 a and a partof the sheltered board 5 which surrounds the upper surface of the mold3; and as a result, the gap 20 is sealed. The gas jetting tool 8 issupplied with a high-pressure gas from a gas supply device (not shown).

A liquid receiving portion 22 having an upper opening is provided on thelower downstream side with respect to the outlet 4 of the mold 3. Theliquid receiving portion 22 receives the cooling medium 18, which hasbeen jetted onto the Mg slab or Mg alloy slab 6, and exhausts it to theoutside.

(Embodiment 2 of an Apparatus for Horizontal Continuous Casting of an MgSlab or Mg Alloy Slab according to the Present Invention)

Shown in FIG. 4 is Embodiment 2 of the apparatus for horizontalcontinuous casting of an Mg slab or Mg alloy slab according to thepresent invention. The apparatus shown in FIG. 4 is provided with asecond gas jetting tool 9 between the outlet 4 of the mold 3 and thecooling medium jetting tool 7. A second sealing gas 24 such as air,inert gas, nitrogen gas, or incombustible gas is jetted from a jettingoutlet 9 a toward the Mg slab or Mg alloy slab 6 drawn out from theoutlet 4 of the mold 3 to thereby form an air curtain or gas curtain 26above the Mg slab or Mg alloy slab. The gas curtain 26 seals the portionabove the Mg slab or Mg alloy slab. In the case where the cooling mediumjetted from the cooling medium jetting tool 7 is liquid, the seal alsocan prevent the liquid, droplets, spray, and the like from running backto the mold side.

(Embodiment 1 of a Method for Horizontal Continuous Casting of an MgSlab or Mg Alloy Slab according to the Present Invention)

In the method for horizontal continuous casting of an Mg slab or Mgalloy slab according to the present invention, the entire upper surfacefrom the portion above the pool 2 in the tundish 1 to the portion abovethe outlet 4 of the mold 3 is covered by the sheltered board 5; themolten Mg or Mg alloy (at 650° C. or more) in the tundish 1 in FIGS. 1Aand 3A is supplied to the mold 3 which is heated by the heater or thelike or is not heated; and the Mg slab or Mg alloy slab undersolidification in the mold 3 is drawn out to the outside from the outlet4 of the mold 3. At the time of the start of drawing-out, as shown inFIG. 1B, the tip end of the molten Mg or Mg alloy under solidification,which is drawn out from the outlet 4 of the mold 3, is coupled with thedummy member 23. The dummy member 23 is drawn out, whereby the Mg slabor Mg alloy slab 6 under solidification is drawn out. Thereafter, thecooling medium 18 is jetted onto the Mg slab or Mg alloy slab 6 from thejetting outlet 7 a of the slit-shaped nozzle, round pipe nozzle, or thelike to cool the Mg slab or Mg alloy slab 6. The cooled Mg slab or Mgalloy slab 6 is continuously drawn out by means of a drawing device 25.Two or more pairs of pinch rolls, which sandwich the Mg slab or Mg alloyslab 6 from both the upper and lower sides to draw out the slab, areappropriately used as the drawing device 25. Water, liquid nitrogen, orthe like is appropriately used as the cooling medium 18.

In this case, the sealing gas 19 such as the inert gas, nitrogen gas, orincombustible gas is jetted into the gap 20 between the free surface 13a of the molten Mg or Mg alloy 13 and the sheltered board 5 surroundingthe upper surface of the mold 3 by means of the gas jetting tool 8provided on the inner side with respect to the outlet 4 of the mold 3,thereby forming the gas curtain 21 in the gap 20. The gas curtain 21seals the outlet 4 side and the inner side of the mold 3. In the case ofthe cooling medium being liquid, the seal prevents oxygen and liquid,droplets, spray, and the like from entering the mold 3 through the gap20. Therefore, it is desirable that the sealing gas 19 be jetted so asto lick (move along) the free surface 13 a. An argon gas or nitrogen gascan be used as the inert gas. A sulfur hexafluoride gas, gas obtained byadding a nitrogen gas to the gas, or the like can be used as theincombustible gas.

As shown in FIG. 4, the air or gas jetting tool 9 is provided betweenthe outlet 4 of the mold 3 and the cooling medium jetting tool 7. Theair, inert gas, nitrogen gas, or incombustible gas is jetted from thejetting outlet 9 a toward the Mg slab or Mg alloy slab 6, therebyforming the air curtain or gas curtain. In the case where the coolingmedium is liquid, it is desirable to prevent the liquid, droplets,spray, and the like from running back to the mold side. That is, suchprevention is preferably achieved by the two curtains, the above curtainand the gas curtain 21 formed by the sealing gas 19 jetted from the gasjetting tool 8. An ordinary compressor may be used as a jetting device.Thus, the usage amount of expensive inert gas, nitrogen gas, orincombustible gas can be vastly saved.

As described above, the molten metal 13, which is supplied to the mold 3kept at a temperature not more than the solidification temperature ofthe casting Mg alloy, first forms a thin solidified shell 13 b with athickness of about 1 to 2 mm along the inner wall surface of the mold 3.Next, the unsolidified molten residue on a portion above the solidifiedshell is drawn out to the outside from the outlet 4 of the mold 3without a fear of breakout while being mounted on the thin solidifiedshell 13 b. The drawn-out Mg slab or Mg alloy slab under solidificationis cooled by the cooling medium to be completely solidified. In thiscase, a drawing speed of the Mg slab or Mg alloy slab 6 can be easilyraised to approximately 16.66 mm/sec. The plate width of the Mg slab orMg alloy slab 6 is adjusted by changing the width dimension of the mold.The plate thickness can be easily adjusted by changing the level of thefree surface 13 a of the molten metal 13 supplied to the tundish 1 andthe mold 3 or changing the depth of the mold 3. The width of the tundish1 on the outlet side is made wider than the width of the inlet of themold 3. Also, the width of the outlet of the mold 3 is made slightlywider than the width of the inlet of the mold 3 (with a taper of 1 to 2degrees). As a result, the width of the Mg slab or Mg alloy slab 6easily becomes stable, and the Mg slab or Mg alloy slab can be moreeasily drawn out to the outside of the mold.

(Embodiment 2 of a Method for Horizontal Continuous Casting of an MgSlab or Mg Alloy Slab of the Present Invention)

With the use of the apparatus for horizontal continuous casting of an Mgslab or Mg alloy slab of the present invention, about 29 kg of an AZ31Bmagnesium alloy for an expanded material was subjected to casting with acasting speed ranging from 1.66 mm/sec. to 16.66 mm/sec. Examinationsand studies were made of the influence of casting conditions on thesurface condition and solidification structure of the obtained AZ31Bslab having a size of a width of 100 mm×a thickness of 5 to 20 mm×a spanof 10 m or more. Further, the relation between mechanical property suchas a tensile strength, proof stress of 0.2%, or elongation and amicrostructure was explained. Moreover, a comparison examination wasperformed between the AZ31B slab and a commercial laminate plate productproduced by another company from the viewpoint of rolling and pressingworkability. As a result, the AZ31B slab, which was manufactured withthe manufacturing method and manufacturing apparatus of the presentinvention, widely differed from a conventional slab, and had a smoothouter circumferential surface as shown in FIG. 6. FIG. 7 shows amacrostructure of a section in a longitudinal direction (castingdirection) of the same slab, and FIG. 8 shows a macrostructure of asection in a width direction (perpendicular direction to the castingdirection) . In addition, FIG. 9 shows a microstructure of the sectionin the longitudinal direction (casting direction) of the same slab, andFIG. 10 shows a microstructure of the section in the width direction(perpendicular direction to the casting direction) . The figures haverevealed that: the AZ31B slab, which is manufactured with themanufacturing method and manufacturing apparatus of the presentinvention, has a solidification structure of which section has a mirrorsurface and in which eutectic crystals of dissolved elements, Al and Zn,are uniformly and minutely distributed in a form of fine particles in Mgprimary crystals serving as basis metal; the slab has no castingdefects, in its inner side, such as holes, gas porosities, segregationof the dissolved elements or impurity elements, and inclusions; and theslab is a high-quality slab having excellent mechanical property andexcellent characteristics such as rolling and pressing workability.

Table 1 is a chemical composition table of an expanded material, AZ31Bwhich is used in the embodiment. Further, FIG. 11 shows an example of amicrostructure of an AZ31B ingot raw material (ingot). TABLE 1 Chemicalcomposition (wt %) Al Zn Mn Si Fe Cu Ni Be Mg AZ31B 3.17 0.92 0.28 0.020.002 0.002 0.0004 0.0005 Bal.

The method for horizontal continuous casting of an Mg slab or Mg alloyslab of the present invention has the following effects because thecooling medium is jetted to the Mg slab or Mg alloy slab, which has beendrawn out from the mold, on the outside and in the vicinity of theoutlet of the mold.

1. Since the temperature of the mold hardly lowers due to the coolingmedium, the temperature of the mold does not need to be raised to atemperature higher than needed. Thus, the lives of the mold and heaterare not shortened extremely, and these components do not need to beexchanged frequently. Further, an excessive current does not need to beflown through the mold heating heater. Thus, the insulation resistancevalues of a heater panel, refractory, and heat insulating material arenot lowered extremely, which does not invite the danger of an electricleakage. Therefore, the Mg slab or Mg alloy slab can be producedcontinuously, thereby being stably supplied. Further, in the case ofbeing cooled at the outlet end of the mold, the Mg slab or Mg alloyslab, which has been drawn out from the outlet of the mold, is cooledimmediately after being drawn out from the outlet of the mold.Accordingly, it does not occur that the slab surface is oxidized to beblack until the slab is cooled after being drawn out from the outlet.

2. The mold temperature does not need to be raised to a temperaturehigher than needed. Thus, the Mg slab or Mg alloy slab in the mold doesnot transform as shown in FIG. 17A. Therefore, there does not arise theproblem in the case of the transformation as shown in FIG. 17A.

3. The slab can be continuously produced for a necessary length.Therefore, there is no limitation on a manufacturing possible length.

4. When the molten metal is rapidly cooled and solidified at arelatively high cooling speed (10 to 10² K/sec.), the Mg slab or Mgalloy slab can be produced which has a microstructure, in which thedissolved elements (Al, Zn) and an intermetallic compound (Mg—Mn) areminutely and uniformly distributed, and which has high corrosionresistance.

According to the method for horizontal continuous casting of an Mg slabor Mg alloy slab of the present invention, the pool in the tundish andthe portion above the mold are sheltered from the surroundings; thesealing gas such as the inert gas, nitrogen gas, or incombustible gas isjetted over the entire width of the free surface of the molten Mg or Mgalloy in the mold to thereby seal the portion above the entire width ofthe free surface; and oxygen and liquid, droplets, spray and the like inthe case of the cooling medium being the liquid, are prevented fromflowing into the mold from the portion. Accordingly, there are providedthe following effects.

1. The air is securely prevented from flowing into the mold. The moltenMg or Mg alloy is sent to the mold from the pool in the tundish, andthus, is shielded against the air until being drawn out from the outlet.Therefore, combustion of the molten Mg or Mg alloy and the Mg slab or Mgalloy slab can be avoided reliably. As a result, the Mg slab or Mg alloyslab, of which surface is not black, can be obtained.

2. The cooling water, spray, and the like can be securely prevented fromflowing into the mold. Thus, there is no danger of explosions, which issafe.

According to the method for horizontal continuous casting of an Mg slabor Mg alloy slab of the present invention, the mold is provided with theheating device such as the heater, and the heating device raises thesurface temperature of the mold to the temperature exceeding the meltingpoint of the casting Mg or the solidification temperature of the Mgalloy. This provides the following effects.

1. The molten Mg or Mg alloy supplied to the mold does not form thesolidified shell along the inner wall surface of the mold, and asolidified interface is formed on the mold. Thus, the crystal grainboundaries are not generated, which are formed from a surface or endportion to an inner portion. Therefore, there can be produced safely andstably the Mg slab or Mg alloy slab with high reliability and highquality which has a smooth surface, which is not accompanied with thegeneration of inner holes, gas porosities, inclusions, and the like dueto solidification shrinkage, and which has no casting defect.Accordingly, the yield is improved, and the reduction of costs can berealized.

2. The mass production of Mg slabs or Mg alloy slabs, which haveexcellent mechanical properties and characteristics that enable easyworking through rolling, pressing, and the like, can be achieved at lowcost.

According to the method for horizontal continuous casting of an Mg slabor Mg alloy slab of the present invention, the surface temperature ofthe mold is kept so as not to exceed the melting point of the casting Mgor the solidification temperature of the Mg alloy; the molten Mg or Mgalloy is supplied to the mold; the thin solidified shell (13 b in FIG.1B) with a thickness of about 1 to 2 mm is formed of the molten metalalong the inner wall surface (bottom surface and both the side surfaces)of the mold; and the unsolidified molten residue on a part above thesolidified shell is drawn out to the outside from the outlet of the moldwhile being mounted on the thin solidified shell. Thus, the followingeffects are also provided.

1. The Mg slab or Mg alloy slab can be drawn out without a fear ofbreakout.

2. The Mg slab or Mg alloy slab, which has been drawn out as in 1described above, can have a ground surface like a mirror surface bygrinding a surface layer (portion of the solidified shell) thereof. Thegrinding amount is much smaller than that of a general slab.

According to the method for horizontal continuous casting of an Mg slabor Mg alloy slab of the present invention, the second sealing gas suchas the air, inert gas, nitrogen gas, or incombustible gas is jetted overthe entire width of the Mg slab or Mg alloy slab, which has been drawnout from the outlet of the mold, between the outlet of the mold and thecooling medium jetting part; the second sealing gas seals the portionabove the Mg slab or Mg alloy slab; and in the case of the coolingmedium being liquid, the seal also prevents the liquid, droplets, spray,and the like from running back to the mold side (with the two steps).Accordingly, the following effects are provided.

1. The above-described various effects of the manufacturing method ofthis application are further improved.

2. The sealing gas jetted in the mold is jetted along the free surfaceof the molten Mg or Mg alloy, and the second sealing gas is jetted alongthe upper surface of the Mg slab or Mg alloy slab. This provides a highsealing effect.

According to the method for horizontal continuous casting of an Mg slabor Mg alloy slab of the present invention, at the time of the start ofdrawing-out the Mg slab or Mg alloy slab, the molten Mg or molten Mgalloy is made to contact with the dummy member, and the Mg slab or Mgalloy slab is drawn out by drawing out the dummy member. Therefore,drawing-out at the time of the start is performed easily.

According to the method for horizontal continuous casting of an Mg slabor Mg alloy slab of the present invention, the mold is provided with theheating element, and the operation of the heating element is madeswitchable between on and off. Thus, the mold can be used both at atemperature not more than the melting point of the casting Mg or thesolidification temperature of the Mg alloy and at a temperatureexceeding the point or temperature. Therefore, the mold can be properlyused in correspondence with applications, which is useful.

The apparatus for horizontal continuous casting of an Mg slab or Mgalloy slab of the present invention is provided with: the shelteredboard which covers the area ranging from the portion above the pool inthe tundish to the portion above the mold outlet to shield the areaagainst the outside; the cooling medium jetting tool which is providedon the outer side of the mold outlet and which jets the cooling mediumonto the Mg slab or Mg alloy slab, which has been drawn out from theoutlet, on the outer side of the mold outlet and in the vicinity of theoutlet to thereby cool the slab; and the gas jetting tool which isprovided on the inner side of the mold outlet and in the vicinity of theoutlet, which jets the inert gas, nitrogen gas, or incombustible gas toseal the portion above the entire width of the free surface, and whichprevents oxygen from flowing into the mold from the outside and alsoprevents the droplets, spray, and the like of the cooling medium fromflowing into the mold. Therefore, the following effects are provided.

1. The molten Mg or Mg alloy and the Mg slab or Mg alloy slab can beshielded from the surroundings with simple manufacturing facilitiesuntil they are drawn out from the outlet of the mold after having beensent to the mold from the pool in the tundish. Thus, the molten metaland slab do not become black through combustion, and can be continuouslymanufactured without explosions. As a result, the Mg slabs or Mg alloyslabs can be supplied stably.

2. Since the molten metal is rapidly cooled and solidified at arelatively high cooling speed (10 to 10² K/sec.), the Mg slab or Mgalloy slab can be produced which has a microstructure, in which thedissolved elements (Al, Zn) and an intermetallic compound (Mg—Mn) areminutely and uniformly distributed, and which has high corrosionresistance.

In the apparatus for horizontal continuous casting of an Mg slab or Mgalloy slab of the present invention, in the case where the mold isprovided with the heating element, the operation of the heating elementis made switchable between on and off. Therefore, the following effectsare provided.

1. The mold can be used both at the temperature not more than themelting point of the casting Mg or the solidification temperature of theMg alloy and at the temperature exceeding the point or temperature.Therefore, the mold can be properly used in correspondence withapplications, which is useful.

2. In the case of 1 described above, when the surface temperature of themold is raised so as to exceed the melting point of Mg or thesolidification temperature of the Mg alloy, the molten Mg or Mg alloysupplied to the mold does not form the solidified shell on the surfaceof the mold, and the solidified interface is formed on the mold. Thus,the crystal grain boundaries are not generated, which are formed from asurface or end portion to an inner portion. Therefore, there can besafely and stably realized the mass production of Mg slabs or Mg alloyslabs with high reliability and high quality each of which has a smoothsurface, which are not accompanied with the generation of inner holes,gas porosities, inclusions, and the like due to solidificationshrinkage, and which have no casting defect. Accordingly, the yield isimproved, and the reduction of costs can be also realized. Besides, theMg slabs or Mg alloy slabs have excellent mechanical properties andcharacteristics that enable easy working through rolling, pressing, andthe like. Further, the molten Mg or Mg alloy supplied from the pool inthe tundish into the mold is not solidified. Thus, the tip end of themolten Mg or Mg alloy can be easily and relatively slowly coupled withthe dummy member with a margin for time at the time of the start ofdrawing-out. Therefore, drawing-out with the dummy member can beperformed easily and reliably.

3. In the case of 1 described above, the surface temperature of the moldis kept so as not to exceed the melting point of the casting Mg or thesolidification temperature of the Mg alloy, and the molten Mg or Mgalloy is supplied to the mold. Resultingly, the thin solidified shell(13 b in FIG. 1B) with a thickness of about 1 to 2 mm is formed of themolten metal along the inner wall surface (bottom surface and both theside surfaces) of the mold, and the unsolidified molten residue on apart above the solidified shell can be drawn out to the outside from theoutlet of the mold while being mounted on the thin solidified shell.Therefore, the Mg slab or Mg alloy slab can be drawn out without a fearof breakout. Moreover, the Mg slab or Mg alloy slab, which has beendrawn out as described above, can have a ground surface like a mirrorsurface by grinding the surface layer (portion of the solidified shell).Therefore, the grinding amount is much smaller than that of a generalslab in which the entire surface of an Mg slab or Mg alloy slab isground.

The apparatus for horizontal continuous casting of an Mg slab or Mgalloy slab of the present invention is provided, between the outlet ofthe mold and the cooling medium jetting tool, with the second gasjetting tool which jets the second sealing gas such as the air, inertgas, nitrogen gas, or incombustible gas on the Mg slab or Mg alloy slabdrawn out from the mold outlet to thereby seal the portion above theentire width of the Mg slab or Mg alloy slab. In the case where thecooling medium is liquid, the seal also prevents the liquid, droplets,spray, and the like from running back to the mold side. Thus, theeffects 1 and 2 are further improved. Further, the second gas jettingtool is provided at the angle at which the second sealing gas is jettedalong the Mg slab or Mg alloy slab. Accordingly, the sealing effect isfurther increased.

The apparatus for horizontal continuous casting of an Mg slab or Mgalloy slab of the present invention is provided, on the outer side ofthe outlet of the mold, with the drawing device that draws out the Mgslab or Mg alloy slab from the outlet. As a result, continuous castingcan be realized.

1. A method for horizontal continuous casting of an Mg slab or Mg alloyslab, comprising: supplying a molten Mg or Mg alloy from a pool in atundish, which is shielded from surroundings, into a mold thatcommunicates with the pool and is shielded from the surroundings;drawing out the Mg slab or Mg alloy slab formed in the mold to anoutside from an outlet of the mold; jetting, after the mold is drawnout, a cooling medium onto the Mg slab or Mg alloy slab on an outer sideof the mold outlet and in a vicinity of the outlet to cool the Mg slabor Mg alloy slab; and jetting a sealing gas over an entire width of afree surface of the molten Mg or Mg alloy on an inner side of the moldoutlet and in the vicinity of the outlet, the gas sealing a portionabove the entire width of the free surface of the molten Mg or Mg alloyin the mold to prevent oxygen from flowing into the mold.
 2. The methodfor horizontal continuous casting of an Mg slab or Mg alloy slabaccording to claim 1, wherein, a surface temperature of the mold is setat a temperature exceeding one of a melting point of Mg and asolidification temperature of an Mg alloy.
 3. The method for horizontalcontinuous casting of an Mg slab or Mg alloy slab according to claim 1,wherein, a surface temperature of the mold is set at a temperature notexceeding one of a melting point of Mg and a solidification temperatureof an Mg alloy such that the molten Mg or Mg alloy supplied from thepool in the tundish into the mold forms a thin solidified shell along aninner wall surface of the mold, and unsolidified molten residue issolidified on the solidified shell to form the Mg slab or Mg alloy slab.4. The method for horizontal continuous casting of an Mg slab or Mgalloy slab according to claim 1, wherein, between the mold outlet and acooling medium jetting part, a second sealing gas is jetted over theentire width of the Mg slab or Mg alloy slab, which has been drawn outfrom the outlet of the mold, the second sealing gas sealing the portionabove the entire width of the Mg slab or Mg alloy slab.
 5. The methodfor horizontal continuous casting of an Mg slab or Mg alloy slabaccording to claim 1, wherein when the Mg slab or Mg alloy slab formedin the mold is to be drawn out from the outlet of the mold, a tip end ofthe molten Mg or molten Mg alloy is made to contact with a dummy memberat a start of drawing-out, the Mg slab or Mg alloy slab is drawn out bydrawing out the dummy member, and after being drawn out, the Mg slab orMg alloy slab is drawn out by a drawing device.
 6. The method forhorizontal continuous casting of an Mg slab or Mg alloy slab accordingto claim 2, wherein when the Mg slab or Mg alloy slab formed in the moldis to be drawn out from the outlet of the mold, a tip end of the moltenMg or molten Mg alloy is made to contact with a dummy member at a startof drawing-out, the Mg slab or Mg alloy slab is drawn out by drawing outthe dummy member, and after being drawn out, the Mg slab or Mg alloyslab is drawn out by a drawing device.
 7. The method for horizontalcontinuous casting of an Mg slab or Mg alloy slab according to claim 3,wherein when the Mg slab or Mg alloy slab formed in the mold is to bedrawn out from the outlet of the mold, a tip end of the molten Mg ormolten Mg alloy is made to contact with a dummy member at a start ofdrawing-out, the Mg slab or Mg alloy slab is drawn out by drawing outthe dummy member, and after being drawn out, the Mg slab or Mg alloyslab is drawn out by a drawing device.
 8. The method for horizontalcontinuous casting of an Mg slab or Mg alloy slab according to claim 4,wherein when the Mg slab or Mg alloy slab formed in the mold is to bedrawn out from the outlet of the mold, a tip end of the molten Mg ormolten Mg alloy is made to contact with a dummy member at a start ofdrawing-out, the Mg slab or Mg alloy slab is drawn out by drawing outthe dummy member, and after being drawn out, the Mg slab or Mg alloyslab is drawn out by a drawing device.
 9. The method for horizontalcontinuous casting of an Mg slab or Mg alloy slab according to claim 1,wherein the sealing gas is jetted toward an outlet side of the moldalong the free surface of the molten Mg or molten Mg alloy.
 10. Themethod for horizontal continuous casting of an Mg slab or Mg alloy slabaccording to claim 2, wherein the sealing gas is jetted toward an outletside of the mold along the free surface of the molten Mg or molten Mgalloy.
 11. The method for horizontal continuous casting of an Mg slab orMg alloy slab according to claim 3, wherein the sealing gas is jettedtoward an outlet side of the mold along the free surface of the moltenMg or molten Mg alloy.
 12. The method for horizontal continuous castingof an Mg slab or Mg alloy slab according to claim 4, wherein the sealinggas is jetted toward an outlet side of the mold along the free surfaceof the molten Mg or molten Mg alloy.
 13. The method for horizontalcontinuous casting of an Mg slab or Mg alloy slab according to claim 5,wherein the sealing gas is jetted toward an outlet side of the moldalong the free surface of the molten Mg or molten Mg alloy.
 14. Themethod for horizontal continuous casting of an Mg slab or Mg alloy slabaccording to claim 6, wherein the sealing gas is jetted toward an outletside of the mold along the free surface of the molten Mg or molten Mgalloy.
 15. The method for horizontal continuous casting of an Mg slab orMg alloy slab according to claim 7, wherein the sealing gas is jettedtoward an outlet side of the mold along the free surface of the moltenMg or molten Mg alloy.
 16. The method for horizontal continuous castingof an Mg slab or Mg alloy slab according to claim 8, wherein the sealinggas is jetted toward an outlet side of the mold along the free surfaceof the molten Mg or molten Mg alloy.
 17. The method for horizontalcontinuous casting of an Mg slab or Mg alloy slab according to claim 4,wherein the second sealing gas is jetted toward a cooling medium jettingside along an upper surface of the Mg slab or Mg alloy slab.
 18. Anapparatus for horizontal continuous casting of an Mg slab or Mg alloyslab, comprising: a sheltered board that covers an area extending from aportion above a pool in a tundish to a portion above a mold outlet toshield the area from an outside; a cooling medium jetting tool that isprovided one of on an outer side of the mold outlet and at an outlet endof the mold and jets a cooling medium onto the Mg slab or Mg alloy slab,which has been drawn out from the outlet, on the outer side of the moldoutlet and in a vicinity of the outlet to cool the slab; and a gasjetting tool having a wide width that is provided on an inner side ofthe mold outlet and jets a sealing gas over an entire width of the freesurface of the molten Mg or Mg alloy to seal a portion above the entirewidth of the free surface of the molten Mg or molten Mg alloy in themold, wherein the sealing gas jetted from the gas jetting tool seals theportion above the entire width of the free surface of the molten Mg ormolten Mg alloy to prevent oxygen from flowing into the mold therefrom.19. The apparatus for horizontal continuous casting of an Mg slab or Mgalloy slab according to claim 18, further comprising a second gasjetting tool provided between the outlet of the mold and the coolingmedium jetting tool, the second gas jetting tool jetting a secondsealing gas over the entire width of the Mg slab or Mg alloy slab thathas been drawn from the outlet of the mold to seal the portion above theentire width of the Mg slab or Mg alloy slab.
 20. The apparatus forhorizontal continuous casting of an Mg slab or Mg alloy slab accordingto claim 19, wherein the second gas jetting tool is provided at an angleat which the second sealing gas is jetted to a cooling medium jettingside along the Mg slab or Mg alloy slab.
 21. The apparatus forhorizontal continuous casting of an Mg slab or Mg alloy slab accordingto claim 18, further comprising, on the outer side of the outlet of themold, a drawing device that draws out the Mg slab or Mg alloy slab. 22.The apparatus for horizontal continuous casting of an Mg slab or Mgalloy slab according to claim 18, wherein the mold comprises a heatingelement.
 23. The apparatus for horizontal continuous casting of an Mgslab or Mg alloy slab according to claim 19, wherein the mold comprisesa heating element.
 24. The apparatus for horizontal continuous castingof an Mg slab or Mg alloy slab according to claim 20, wherein the moldcomprises a heating element.
 25. The apparatus for horizontal continuouscasting of an Mg slab or Mg alloy slab according to claim 21, whereinthe mold comprises a heating element.
 26. The apparatus for horizontalcontinuous casting of an Mg slab or Mg alloy slab according to claim 18,wherein the mold comprises no heating element.
 27. The apparatus forhorizontal continuous casting of an Mg slab or Mg alloy slab accordingto claim 19, wherein the mold comprises no heating element.
 28. Theapparatus for horizontal continuous casting of an Mg slab or Mg alloyslab according to claim 20, wherein the mold comprises no heatingelement.
 29. The apparatus for horizontal continuous casting of an Mgslab or Mg alloy slab according to claim 21, wherein the mold comprisesno heating element.
 30. The apparatus for horizontal continuous castingof an Mg slab or Mg alloy slab according to claim 18, wherein, in a casewhere the mold comprises the heating element, the heating element ismade switchable between on and off.
 31. The apparatus for horizontalcontinuous casting of an Mg slab or Mg alloy slab according to claim 19,wherein, in a case where the mold comprises the heating element, theheating element is made switchable between on and off.
 32. The apparatusfor horizontal continuous casting of an Mg slab or Mg alloy slabaccording to claim 20, wherein, in a case where the mold comprises theheating element, the heating element is made switchable between on andoff.
 33. The apparatus for horizontal continuous casting of an Mg slabor Mg alloy slab according to claim 21, wherein, in a case where themold comprises the heating element, the heating element is madeswitchable between on and off.
 34. The apparatus for horizontalcontinuous casting of an Mg slab or Mg alloy slab according to claim 22,wherein, in a case where the mold comprises the heating element, theheating element is made switchable between on and off.
 35. The apparatusfor horizontal continuous casting of an Mg slab or Mg alloy slabaccording to claim 23, wherein, in a case where the mold comprises theheating element, the heating element is made switchable between on andoff.
 36. The apparatus for horizontal continuous casting of an Mg slabor Mg alloy slab according to claim 24, wherein, in a case where themold comprises the heating element, the heating element is madeswitchable between on and off.
 37. The apparatus for horizontalcontinuous casting of an Mg slab or Mg alloy slab according to claim 25,wherein, in a case where the mold comprises the heating element, theheating element is made switchable between on and off.
 38. The apparatusfor horizontal continuous casting of an Mg slab or Mg alloy slabaccording to claim 26, wherein, in a case where the mold comprises theheating element, the heating element is made switchable between on andoff.
 39. The apparatus for horizontal continuous casting of an Mg slabor Mg alloy slab according to claim 27, wherein, in a case where themold comprises the heating element, the heating element is madeswitchable between on and off.
 40. The apparatus for horizontalcontinuous casting of an Mg slab or Mg alloy slab according to claim 28,wherein, in a case where the mold comprises the heating element, theheating element is made switchable between on and off.
 41. The apparatusfor horizontal continuous casting of an Mg slab or Mg alloy slabaccording to claim 29, wherein, in a case where the mold comprises theheating element, the heating element is made switchable between on andoff.
 42. The method according to claim 1 wherein the cooling medium is aliquid, and the jetting of the sealing gas seals the portion above theentire width of the free surface of the molten Mg or Mg alloy in themold to prevent any liquid or vapor substance that is generated due tothe jetting liquid from returning to the mold.
 43. The method accordingto claim 42, wherein the liquid or vapor substance includes at least oneof liquid, droplets, spray and vapor.
 44. The method according to claim4, wherein the cooling medium is a liquid, and the jetting of thesealing gas seals the portion above the entire width of the free surfaceof the molten Mg or Mg alloy in the mold to prevent any liquid or vaporsubstance that is generated due to the jetting liquid from returning tothe mold.
 45. The method according to claim 44, wherein the liquid orvapor substance includes at least one of liquid, droplets, spray andvapor.
 46. The apparatus according to claim 18, wherein the coolingmedium is a liquid, and the jetting of the sealing gas seals the portionabove the entire width of the free surface of the molten Mg or Mg alloyin the mold to prevent any liquid or vapor substance that is generateddue to the jetting liquid from returning to the mold.
 47. The apparatusaccording to claim 46, wherein the liquid or vapor substance includes atleast one of liquid, droplets, spray and vapor.
 48. The apparatusaccording to claim 19, wherein the cooling medium is a liquid, and thejetting of the sealing gas seals the portion above the entire width ofthe free surface of the molten Mg or Mg alloy in the mold to prevent anyliquid or vapor substance that is generated due to the jetting liquidfrom returning to the mold.
 49. The apparatus according to claim 48,wherein the liquid or vapor substance includes at least one of liquid,droplets, spray and vapor.